1. Field of the Invention
The present invention relates generally to an optical system which can be used as an ocular or image-forming system, and more particularly to an optical system which is reduced in terms of both curvature of field and coma and has an improved resolution even at a wide field angle and a large pupil diameter.
2. Discussion of Related Art
The applicant has already filed Japanese Patent Application No. 5(1993)-264828 for an optical system which enables curvature of field and coma to be almost completely corrected by filling vitreous material or the like between at least two semitransparent curved surfaces to facilitate its production, and increasing the distance between two semi-transparent concave surfaces. Even with the optical system disclosed in the above patent application, however, curvature of field and coma become noticeable when the angle of view is as wide as about 100.degree. and the pupil diameter is as large as about 100 mm.
An account will now be given of why off-axis aberrations are produced in the optical system disclosed in Japanese Patent Application No. 5-264828 with reference to an optical path diagram shown in FIG. 4.
In FIG. 4, reference numeral 1 represents a semi-transparent concave surface located in the vicinity of a pupil plane E and 2 a semitransparent concave surface located in the vicinity of an image plane I. A beam of rays is transmitted through or reflected at each surface in the order of 1-1, 2-1, and 2-2. The beam of rays is subjected to refraction at 1-1 and 2-2, and to reflection at 1-2 and 2-1.
How to compensate for coma will first be explained. To reduce or substantially eliminate the occurrence of coma at the image plane I, it is important to reduce the quantity of coma occurring at 1-1 and 2-1 where a beam of off-axis rays is first incident on the respective semitransparent surfaces 1 and 2. The optical axis length from the pupil E to the surface 1 is shorter than that from the pupil 1 to the surface 2. To reduce the angle of incidence of the beam of off-axis rays at 1-1 and 2-1 and reduce the difference in the angle of incidence between an off-axis marginal ray above the optical axis and an off-axis marginal ray below the optical axis, thereby reducing the quantity of coma produced, it is required that the surface 1 be stronger in power than the surface 2 and a curvature difference be between the surfaces 1 and 2.
To reduce the quantity of coma occurring at 1-2 and 2-2 where the beam of off-axis rays are again incident on the respective semitransparent surfaces, too, the power of the surface 1 should be stronger than that of the surface 2. This is because when the power of the surface 1 is too weak, the angle of inclination of the normal of the surface 1 becomes small with respect to the optical axis; so the angle of incidence of an off-axis chief ray at 1-2 becomes large. As a result, there is a large difference in the angle of incidence between the marginal rays above and below the optical axis, which otherwise gives rise to some considerable coma. It is here noted that the image-forming system disclosed is an almost telecentric one on the image side. When the power of the surface 2 is too strong, it is located nearly parallel to the optical axis; so the angle of an off-axis chief ray incident on 2-2 becomes large, resulting in a large difference in the angle of incidence between the marginal rays above and below the optical axis and hence giving rise to some considerable coma. To reduce the quantity of coma occurring at 1-2 and 2-2, it is then required that the power of the surface 1 be strong and the power of the surface 2 be weak.
Reference will now be made to where curvature of field takes place primarily and how to compensate for it. The largest curvature of field is negative curvature of field produced upon reflection at 1-2, and positive curvature of field produced upon reflection at 2-1. To inhibit the occurrence of curvature of field at the image plane 1, it is required that uniform power be imparted to the surfaces 1 and 2, so that the negative curvature of field produced upon reflection at the surface 1 and the positive curvature of field upon reflection at the surface 2 can be offset each other.
In the optical system disclosed in Japanese Patent Application No. 5-264828, how to correct coma is in contradiction to how to correct curvature of field, as already mentioned. In other words, it is difficult to correct both aberrations at the same time. If the field angle and pupil diameter are not particularly large, it is then possible to correct curvature of field and coma by placing the powers of both semitransparent concave surfaces 1 and 2 in a well balanced state. When the field angle and pupil diameter are large, however, difficulty is involved in preventing curvature of field and coma from becoming worse.